a) Field of the Invention
The present invention relates to a machine and method for boring tunnels in the earth strata, and more particularly to such a machine and method which is designed to operate in geological ground conditions where converging conditions exist, such as may exist when a tunnel is being bored at substantial depths below the earth's surface.
There are in the prior art tunnel boring machines which have a forward rotating cutterhead that engages the tunnel end surface to bore into the earth strata, as the machine progresses forwardly into the tunnel. Quite commonly, the machine is moved through the tunnel by means of gripper shoes which are pressed outwardly from the main body of the machine to engage the tunnel side wall and move the machine forwardly from these gripper shoes. The rock fragments and other debris resulting from the boring operation are collected by the rotating cutterhead and deposited on a conveyor to be carried to the rear part of the machine for removal from the tunnel.
As the tunnel is being dug, it is usually necessary to provide support structure for the tunnel side wall, and this is commonly done by providing support rings which are placed circumferentially within the tunnel wall, with these being spaced at longitudinal intervals in the tunnel. Then reinforcing members or material is placed between the rings, followed by placing shotcrete or other structural material against the tunnel side wall.
When a tunnel is being dug at substantial depth below the earth surface, even though the earth strata through which a tunnel is being dug is relatively stable, because of the substantial pressures existing at that depth, the tunnel wall tends to converge. For example, as the cutting head advances through the tunnel, the side wall could "creep" or "flow" radially inwardly. While some of this convergence of the tunnel wall is inevitable at substantial depths below the earth's surface, it is desirable that this be minimized as much as possible.
Also, there are sometimes other geological conditions that dictate that adequate support should be placed in the tunnel side wall as close to the cutting head as is practical, so that the earth strata immediately after the cutterhead be properly supported.
It is toward problems such as these, (and others associated therewith) that the present invention is directed. More specifically, the present invention is particularly directed toward the problem of digging relative large diameter tunnels, such as railroad tunnels, which are to extend through the earth strata at rather substantial depths.